Recently, as liquid crystal display devices have become thinner and larger, the thickness of parts used for them is becoming thinner and thinner. A liquid crystal display (“LCD”) is equipped with a backlight, which is a light emitting part that emits light from the back of LCD, and a light guide plate or a diffusion plate for diffusing or transmitting light is used depending on the type and position of light source. The thickness of the light guide plate also becomes thinner and thinner in accordance with the recent trend, and the general level of the light guide plate actually used is around 0.5 mm in thickness, but the thinnest is up to about 0.3 mm, and the thickness tends to become thinner in future.
In accordance with the thinning trends, the use of edge type backlight unit in which LEDs are mounted to the corner of the backlight is increasing, instead of cold cathode fluorescent lamps (CCFLs) which have been mainly used in the past. In the edge type backlight unit, light emitted from a light source mounted to the corner is transmitted through the light guide plate, and part of the light transmitted through the plate is scattered by a light scattering layer applied to the surface of the plate, whereby the liquid crystal display device is illuminated by the surface light source that uniformly emits light on the entire surface. The light scattering layer is formed by transferring or printing a dot pattern on the surface of the light guide plate. Recently, a fine prism structure is also transferred to increase the light efficiency.
Since the light guide plate requires high light transmittance, PMMA, which is an acrylic resin, is generally used as a material of the light guide plate. Although acrylic resins have high light transmittance, they are insufficient in mechanical strength and thus are not suitable for application to a thin light guide plate, and also they have insufficient heat resistance and thus are vulnerable to heat generated in electronic equipment, which are disadvantageous.
Polycarbonate has attracted attention instead of such an acrylic resin. Polycarbonate is superior in mechanical strength as compared with acrylic resin and thus can be used as a material for a thin light guide plate. In addition, since polycarbonate is excellent in heat resistance and flame retardancy, it is gradually replacing the acrylic resin in LED backlight unit and lighting apparatus with a large amount of heat generation. However, since polycarbonate has a lower total light transmittance than an acrylic resin, it is required that polycarbonate has a light transmittance corresponding to that of an acrylic resin while maintaining the advantages of polycarbonate.
In this connection, Japanese Patent Laid-Open Publication No. 2008-045131 discloses that PMMA having a viscosity average molecular weight (Mv) of 20,000 to 60,000 among an acrylic resin is blended in a range of 0.1 to 0.3 phr with a polycarbonate having an Mv of 15,000 to 40,000 to exhibit excellent photoconductivity. However, it is still necessary to improve physical properties such as heat resistance.
Given the above circumstances, the present inventors have conducted intensive studies about a material which can be used as a material for the light guide plate, and found that a polycarbonate resin composition comprising a block copolymer containing polyethylene oxide block and polypropylene oxide block and a vinyl-based polymer including a (meth)acrylate repeating unit containing an epoxy functional group at the same time, as described below, is excellent in light transmittance, color tone stability and the like, and thus can be used as a material for a light guide plate, thereby completing the present invention.